Doctopus is a Google Sheets add-on that can be used to collect and organize documents (most likely from student work being submitted). It works closely with Google Classroom and can pull work in directly from classroom and display in a Google Sheet. Goobric is a Chrome extension that allows you to embed a rubric you design in Google Sheets to any document, thereby making grading that much easier.

Turn your Google Spreadsheets into flashcards, quizzes, games and more!

This is a Google Forms grading add-on tool. It utilizes a teacher-created key to grade, generate reports and distribute grades to students via email.

This is a tool that functions like a mail merge. You can set it a rubric in a google form, and the results will publish to a Google Sheet. You can then set up mail merge fields in a Google Doc feedback report to students. You can set it up so that once you complete the form rubric it automatically emails the report to students.